Apple built OSX from scratch, sure. Then they patched it .1 .2 .3 .4 .5 .6, and all of the releases in between. And charges a pile of money to get the patches.
On a similar note, Mocrosoft does rebuild it's OS but keeps the same kernel (NT-XP-Vista). And all of the patches (Read: Service packs, which are really on par with OSX Point releases) are free.